The capacities to introduce a particular foreign or native gene sequence into a mammalian cell and to control the expression of that gene are of substantial value in the fields of medical and biological research. Such capacities provide a means for studying gene regulation, and for designing a therapeutic basis for the treatment of disease.
The introduction of a particular foreign or native gene into a mammalian host cell is facilitated by introducing a gene sequence into a suitable nucleic acid vector. A variety of methods have been developed which are capable of permitting the introduction of such a recombinant vector into a desired host cell. In contrast to methods which involve DNA transformation or transfection, the use of viral vectors can result in the rapid introduction of the recombinant molecule in a wide variety of host cells. In particular, viral vectors have been employ ed in order to increase the efficiency of introducing a recombinant nucleic acid vector into host cells. Viruses that have been employed as vectors for the transduction and expression of exogenous genes in mammalian cells include SV40 virus (see, e.g., H. Okayama et al., Molec. Cell Biol. 5, 1136-1142 (1985)); bovine papilloma virus (see, e.g., D. DiMalo et al., Proc. Natl. Acad. Sci. USA 79, 4030-4034 (1982)); adenovirus (see, e.g., J. E. Morin et al., Proc. Natl. Acad. Sci. USA 84, 4626 (1987)), adeno-associated virus (AAV; see, e.g., N. Muzyczka et al., J Clin. Inveit. 94, 1351 (1994)); herpes simplex virus (see, e.g., A. I. Geller, et al., Science 241, 1667 (1988)), and others.
Retrovirus-based vectors are particularly favored as tools to achieve stable, integrated gene transfer of foreign genes into mammalian cells. Retroviruses that have been employed as vectors for the introduction and expression of exogenous genes in mammalian cells include the Moloney murine sarcoma virus (T. Curran et al., J. ViroL. 44, 674-682 (1982); A. Gazit et al, J Virol. 60, 19-28 (1986)) and murine leukemia viruses (MuLV; A. D. Miller, Curr. Tsp. Microbiol. Immunol. 158, 1-24 (1992).
Efforts to introduce recombinant molecules into mammalian cells have been hampered by the inability of many cells to be infected by the above-described viral or retroviral vectors. Limitations on retroviral vectors, for example, include a relatively restricted host range, based in part on the level of expression of the membrane protein that serves as the viral receptor. M. P. Kavanaugh et al., Proc. Natl. Acad. Sci USA 91, 7071-7075 (1994). Other limitations include the inability to integrate into non-dividing cells (e.g., neurons, hepatocytes, myofibers, hematopoictic stem cells), modest vector titers available with current packaging systems, and the fragility of vector particles that precludes purification and concentration.
Lentiviruses are a subgroup of retroviruses that are capable of infecting non-dividing cells. L. Naldini et al. report a lentiviral vector system based on the human immunodeficiency virus (HIV) that is capable of transducing heterologous gene sequences into non-proliferative HeLa cells and rat fibroblasts, as well as into human primary macrophages and terminally differentiated neurons. Science 272, 263-267 (1996). However, the use of such a system in humans raises serious safety concerns, due to the possibility of recombination by the vector into a virulent and disease-causing form.
Accordingly, a need remains for a safe and efficient lentiviral vector systems capable of mediating gene transfer into a broad range of dividing and non-dividing cells.